AgentK
The poor man’s Raiden will never be apart of the main roster until Raiden is out of the spotlight (never), or until we get another MKA-type of game where they have no choice but to include filler bores like Fujin, Kobra, Darrius etc.
Yeah, this is basically it. His role was Earthrealm Guardian. Raiden fills that role and is infinitely more interesting and iconic. There's no reason to include him while Raiden is a central part of the story. It would be redundant.
That being said, if I'm NRS, I'm getting mighty sick of fans demanding Fujin all the time, so I just do him as DLC. Or when Raiden is sidelined for a game, Fujin can take a shot and then get wiped out only for Raiden to return.